Distribution Uniformity (DU_{LQ)} is one way to measure the uniformity of coverage by a statistical parameter. LQ equals low quarter. The DU_{LQ} depends on distribution characteristics or spray pattern of the specific type of sprinkler and the spacing of the sprinklers. Other factors including wind direction, wind speed, water pressure and stream deflection at the nozzle can influence the distribution pattern.
The audit procedure to be used in this module calculates the DU_{LQ} using the smaller amounts of water captured and the average amount of water captured in catch cans in a pattern under the sprinkler system. The calculation uses the quarter of the lower amounts captured to compare to the average of the total sample
The DU_{LQ}is defined as the average water applied in 25% of the area receiving the least amount of water, regardless of location, divided by the average water applied over the total area. The water is captured in containers spaced out in an even pattern under the sprinkler system inside a square or rectangular formed by four sprinklers covering the area. A perfectly uniform application would give a DU of 100%. In a landscape system an average DU of 55% to 75% is common; some will be lower.
For the grower or irrigator, the distribution uniformity gives guidance to the amount of water to apply in order to ensure that the entire crop receives the required water. Thus, if the crop needs an application of one inch of water, some areas will receive less than others. A DU_{LQ}= 70% indicates that to get 1 inch of water onto the ground, 1 inch divided by 0.70 equals 1.42 inches of water to be applied to ensure all areas get at least one inch of water.
DU = (Average volume of depth of lower quarter / Average volume or depth of all) x 100
Christiansen’s Coefficient of Uniformity (CU) is another means of calculating uniformity and it will be discussed near the end of this module when data analysis is discussed. It differs in its method of calculation. The average value of all water depths or volumes is subtracted from each test value and the absolute (positive value) of all these deviations are then averaged. This will be covered again later.
CU = 100[1 – (average deviation / average depth or volume)]
Scheduling Coefficient (SC) is a run time multiplier. The average depth of the application is divided by the single low “dry window” or lowest depth or volume. Basically, this tells the grower how much extra water must be applied (overirrigation) to ensure the dry area is wetted enough.
SC = average depth or volume / lowest depth or volume (dry area)
Rain Bird Corporation discusses Distribution Uniformity and illustrates the root zone wetting patterns for different degrees of uniformity. The irrigation efficiency and economic effects are also discussed in this article. This discussion applies to both inground and container crops. For container crops, the excessive water is runoff.
Rain Bird also has a Tech Tip sheet on Sprinkler Irrigation Uniformity that illustrates their SPACE program for depending uniformity. Graphs of sprinkler distribution are shown as well as the DU and CU uniformity values for three sprinkler layouts.
It is important to know the distribution uniformity of your system in order to schedule irrigation properly or to evaluate the need for making improvements.
Overhead and drip (trickle) irrigation systems will be considered. Pressure is an energy term to be considered as well as its effect on selecting pipe sizes.
